This invention relates to radio receivers and particularly but not exclusively to such receivers forming part of the transceiver of a portable radio telephone or handset, for communication with a base station in a radio telephone system. The invention also relates to a method of operation of such radio receivers. The invention also relates, particularly but not exclusively, to handsets for communication with base stations in a digital cellular radio telephone system employing transmission by a plurality of carrier frequencies in frames, such as TDMA frames, each consisting of a predetermined number of time slots.
Systems operating under the same protocol can cause problems if their spheres of influence overlap. This is because signals from a user on one system may occur at the same time as a signal from a user of the other system. When the two systems are unsynchronised, this pattern of possible interference is not fixed, instead, as the timing of one system will drift relative to the other, the interference pattern will differ with time. A pair of communication channels utilised on respective systems can xe2x80x98slidexe2x80x99 in and out of interference with one another over a period of time. Such interference is known as a xe2x80x98sliding collisionxe2x80x99.
FIGS. 1a-1d illustrate a sliding collision. In FIG. 1a, bursts 1 and 2 from system 1 and system 2, respectively, do not overlap. There is, therefore, no interference i.e. no xe2x80x98collisionxe2x80x99. As the bursts 1 and 2 drift, one relative to the other, the two bursts 1 and 2 come closer (FIG. 1b) and begin to interfere 3 (FIG. 1c). As time passes, interference progressively increases (FIG. 1d).
The sliding collision illustrated in FIG. 1 results in the burst 1 of system 1 losing data at the back end of the burst, while the burst 2 of system 2 loses information from the front of the burst. This type of corruption of a signal has the potential to be very damaging to signal quality and should, therefore, be detected as quickly as possible so that a new channel can be selected. Sliding collisions are difficult to avoid as quickly as might be desirable as, in order to prevent thrashing i.e. frequent or continual hopping of channels, there is normally a delay or hysteresis before a determination to change (handover) channels is made for any detected errors. This prevents temporary deteriorations in channel quality from precipitating channel handover. Such temporary channel deteriorations would typically be caused by fast fades.
In accordance with the present invention there is provided a radio receiver for communication on a selected channel of a radio telephone system in which information is transmitted in bursts, each burst having respective portions independently capable of indicating burst errors, one of the error indicating portions being disposed towards one end of the burst, wherein the radio receiver comprises processing means operable to select another channel for communication dependent upon the distribution of errors between the respective error indicating portions.
The processing means may be operable to select another channel dependent upon a distribution of errors in which the one of the error indicating portions is indicative of an error absent a similar indication from another of the respective error indicating portions. Such an error distribution may be indicative of a sliding collision. Verification of the error pattern over a number of bursts may, however, be desirable before another channel is selected.
By comparing errors detected by respective portions of the burst, sliding collisions can quickly be distinguished from fast fades or other temporary causes of signal deterioration. This allows information loss due to a sliding collision to be avoided by handing over to a new channel without the in-built hysterisis otherwise required before a fast fade can be distinguished from a permanent signal deterioration.
It may be desirable to make a determination that an error pattern is the result of a sliding collision only following at least two successive bursts showing the same pattern of errors in order to distinguish a sliding collision from other temporary signal deteriorations, for example the start or end of a fast fade affecting the burst.
Handover following an error distribution pattern indicative of a sliding collision may be delayed for a predetermined number of bursts. The processing means may then be operable to handover at the end of the period of delay if the distribution of errors still indicate a sliding collision. This gives one of the handsets communicating on the channel on which a sliding collision is occurring the opportunity to handover, leaving the communication channel free from interference for use by the other handset. If one handset hands over, the error distribution pattern for the other handset will subsequently indicate no interference requiring handover unless the channel has deteriorated for other reasons in which case selection of another channel would no longer be necessary for the xe2x80x98otherxe2x80x99 handset.
The one of the error detecting portions is preferably a guard portion at one extreme of the burst protecting the information carried by the burst.
The one of the error detecting portions may be at the front or rear of a burst. Preferably there is an error detecting portion towards each end of the burst allowing sliding collisions occurring from each direction to be detected efficiently.
If the portions at each end of a burst are guard portions it can be several frames before data is lost as a result of a sliding collision. This is because there is typically some regulation on clocking frequency for independent systems which means sliding collisions are generally slow, giving time to enable a more certain determination of a sliding collision to be made before handover is attempted.
The second portion may be a separate error detection string within the burst or a portion towards the opposite end of the burst. It is possible for more than one error detection field to be utilised in determining if an error is the result of a sliding collision.
In accordance with a second aspect of the invention there is provided a method of operation of a radio handset for communication on a radio telephone system in which information is transmitted in bursts, each burst having respective portions independently capable of indicating burst errors, one of the error portions being disposed towards one end of the burst, comprising selecting another channel for communication dependent upon the distribution of errors over the respective error indicating portions.